The present invention relates generally to a hydraulic actuator and, more particularly, to an improved hydraulic actuator for use in an active mount for a vibrating component in a system for reducing vibration and noise transmission from the vibrating component to a support structure.
Hydraulic actuators are used in numerous environments to induce movement of one object with respect to another object. A hydraulic actuator generally includes a cylinder and a moveable piston inside the cylinder. A piston rod is connected to the piston and extends outwardly from one end of the cylinder where the rod end is attached to the first object. The other end of the cylinder is mounted, directly or indirectly, to the second object. The means for mounting the piston rod and cylinder to the objects may incorporate flexible bearing assemblies to provide some xe2x80x9csoftnessxe2x80x9d to the attachment to allow for possible misalignment. Such bearing assemblies preferably comprise elastomeric material. Pressurized hydraulic fluid is introduced into the interior of the cylinder on one or both sides of the piston to effect longitudinal movement of the piston in the cylinder so that the objects are moved relative to one another.
Hydraulic actuators may be used as a component of an active mount in a system for reducing vibration and noise transmission from a vibrating component to a support structure. For example, hydraulic actuation systems are used for actively reducing the vibratory and acoustic loads on aircraft, particularly rotary wing aircraft such as helicopters. A primary source of vibratory and acoustic loads in a helicopter is the main rotor system. The main rotor system of a helicopter includes rotor blades mounted on a vertical shaft that projects from a transmission, often referred to as a gearbox. The gearbox comprises a number of gears that reduce the rotational speed of the helicopter""s engine to the much slower rotational speed of the main rotor blades. The gearbox has a plurality of mounting xe2x80x9cfeetxe2x80x9d which are connected directly to structure in the airframe that supports the gearbox. The main rotor lift and driving torque produce reaction forces and moments on the gearbox. All of the lift and maneuvering load torques are passed from the main rotor blades to the airframe through the mechanical connection between the gearbox feet and the airframe. The airframe structure that supports the gearbox is designed to react to these primary flight loads and safely and efficiently transmit the flight loads to the airframe.
In addition to the nearly static primary flight loads, the aircraft is also subjected to vibratory loads originating from the main rotor blades and acoustic loads generated by clashing of the main rotor transmission gears. The vibratory and acoustic loads produce vibrations and audible noise that are communicated directly to the helicopter airframe via the mechanical connection between the gearbox and the airframe. This mechanical connection thus becomes the xe2x80x9centry pointxe2x80x9d for the unwanted vibration and noise energy into the helicopter cabin. The vibrations and noise within the aircraft cabin cause discomfort to the passengers and crew. In addition, low frequency rotor vibrations are a primary cause of maintenance problems in helicopters.
Active vibration and noise reduction systems in aircraft utilize sensors to monitor the status of the aircraft, or the vibration producing component, and a computer-based controller to command actuators to reduce the vibration and noise. The sensors are located throughout the aircraft and provide signals to the adaptive controller. The controller provides signals to the hydraulic actuation system, including a plurality of actuators located at strategic places within the aircraft. The actuators produce controlled forces or displacements that attempt to minimize vibration and noise at the sensed locations.
Two methods of actuator placement are frequently used in the active system: (1) distribution of actuators over the airframe, or (2) co-location of the actuators at, or near, the vibration or noise entry point. When applied to the main rotor system of a helicopter, the co-location approach places the actuators at or near the structural interface between the transmission and airframe stopping vibration and noise near the entry point before it is able to spread out into the aircraft. This has the advantage of reducing the number of actuators and the complexity of the control system. Active systems using co-location to counteract vibration and noise employ actuators mounted in parallel (across) or in series (between) the transmission gearbox feet and airframe support structure.
When the actuator is mounted in series with the vibrating component and its support structure, six possible degrees of freedom exist between the two objects. However, only the degree of freedom along the principle load-carrying axis is actively controlled for vibration and noise reduction. The remaining degrees of freedom must remain unconstrained to prevent vibration and noise from reaching the support structure. The longitudinal axis of the actuator is aligned with the principle load carrying axis. Further, since the elastomeric bearing is located between the piston rod and the attachment point to the vibrating component, the bearing must provide high static and dynamic stiffness along this active, load-carrying axis so that motions of the piston translate directly into unattenuated motions at the attachment point. To ensure that motions at the attachment point along the five non-active degrees of freedom do not create vibration and noise, the stiffness between the attachment point and actuator along these directions must be low. However, the need for the elastomeric bearing to be stiff along its principle load-carrying axis, yet soft about the other five degrees of freedom, can cause the elastomeric bearing to be unstable under load. Also, transverse and rotational motions at the attachment point become transverse and rotational forces through the stiffness of the elastometric bearing. These forces are transmitted to the piston and can induce high loads between the piston and cylinder that may cause the piston to bind.
Moreover, since hydraulic actuators operate under high pressure, leakage of hydraulic fluid often occurs. This leads to maintenance problems as well as environmental concerns. Additionally, escaping hydraulic fluid can damage the elastomeric material of the bearing.
Examples of conventional seals used between a piston and a cylinder include elastomeric seals, spring-energized seals, and piston rings. Elastomeric seals tend to wear rapidly in actuators serving as active mounts as the result of excessive friction between the elastomeric material and the cylinder mating surface. The friction may be characterized as xe2x80x9cinterlocking,xe2x80x9d which increases with roughness of a mating surface, and xe2x80x9cadhesive,xe2x80x9d which increases with an increase in the contact area with the mating surface. Even when the smoothness of an elastomeric seal""s mating surface is increased, the resulting decrease in interlocking friction is insufficient to offset the increase in the adhesive friction. A low friction seal is vital to reducing noise and vibration. Since the piston will move within the cylinder at the frequencies of the noise and vibration, a high seal friction will partially regenerate the shaking forces in the cylinder wall that are desired to be reduced. This is especially true for noise, which is characterized by already small disturbance forces.
Conventional spring-energized seals include a U-shaped jacket, often made of a low-friction polymer, and a U-shaped metal spring device disposed in the jacket. While the friction between the seal and the cylinder is low, in many actuator applications where transverse loads are applied to the piston rod, the seal stiffness is inadequate to prevent the piston from contacting the cylinder. When the piston contacts the cylinder, the cylinder can become abraded. The abrasions in the cylinder increase the wear rate of the seal, leading to premature failure of the seal and the actuator. Supporting the piston rod at one or two points along the rod""s axis can prevent the piston from contacting the cylinder. For example, journal bearings through which the piston rod passes are routinely used for this purpose. However, a hydraulic actuator used for vibration control must have five unconstrained degrees of freedom. The use of any form of piston rod support, such as journal bearings, would reduce the number of degrees of freedom to two: translation along the piston rod""s axis and rotation about that axis.
Piston rings are effective long-life seals but due to their high leakage rate are never used to support static loads, which constitute the majority of the load seen by an actuator in a helicopter application.
The size and weight of the actuator are also important to optimize performance. In airborne systems this is self-apparent. Large actuators increase the weight of the system, and accordingly, power consumption, when used on a helicopter. Additionally, the desire to place the actuator in series with a vibrating object and its support structure dictates a small size. The need for a small size is governed by the physics of vibration isolation using series actuators. All support structures have a characteristic stiffness that is presented to the vibrating object. Generally the support structure""s stiffness is fixed, or can only be changed at great expense. To effectively isolate the vibrating object from the support structure, the stiffness presented to the vibrating object by the series actuator must be less then the support structure""s own stiffness. Thus the actuator""s stiffness must be measured relative to the supporting structure""s stiffness. While airframes are very strong, they are not very stiff, as stiffness is generally gained via added weight. A small actuator can be placed closer to the interface between the vibrating object and the support structure, reducing the need for and length of support and mounting brackets. The use of such brackets has the effect of reducing the already low stiffness of the support structure. This in turn drives the actuator stiffness to be lower still. There are limits to how low the actuator stiffness can be made and still provide a stability margin against buckling under load.
For the foregoing reasons, there is a need for an active mount including a hydraulic actuator comprising a low-friction seal that minimizes leakage of hydraulic fluid from the cylinder and has adequate stiffness to maintain clearance between the piston and the cylinder when subjected to transverse loads. The new hydraulic actuator should withstand, without buckling, the significant loads generated when used in an active mount for the transmission of the main rotor system of a helicopter. Further, any transverse or rotational motions on the actuator should not induce high loads between the piston and the housing. A compact, relatively lightweight actuator is desirable in order to maximize reduction of noise and vibration transmission.
Therefore, it is an object of the present invention to provide a hydraulic actuator including a low-friction seal that minimizes leakage of hydraulic fluid and has adequate stiffness to maintain clearance between the actuator""s piston and cylinder when subjected to transverse loads.
A further object of the present invention is to provide a hydraulic actuator that is compact and relatively lightweight, and when incorporated into an active mount system provides maximum reduction of noise and vibration transmission between a vibrating component and a support structure.
It is also an object of the present invention to provide a hydraulic actuator that does not induce high loads between the piston and cylinder due to transverse motions and rotations at the point of attachment.
It is a further object of the present invention to provide a hydraulic actuator that does not cause the piston to bind in the cylinder due to transverse motions and rotations at the point of attachment.
It is still further an object of the present invention to provide a hydraulic actuator that does not buckle when subjected to high axial loads.
According to the present invention, a seal assembly is provided for use in sealing a void between two relatively moveable coaxial members, a hollow first member having a cylindrical inner surface, and a second cylindrical member movably disposed in the first member. The seal assembly is disposed in a groove in the second member, and includes an annular seal member comprising the polymer polytetrafluoroethylene (PTFE) resin. The inner edge of the seal member is disposed in the groove, and the seal member""s outer edge contacts the inner surface of the first member to form a seal between the members. The seal assembly also includes backing rings on each side of the seal, and on one side, an annular compression spring in the groove between one of the backing rings and one edge of the groove. The spring exerts force axially to compress the seal member, impelling the seal member to increase in dimension radially and maintain the seal between the members.
Also according to the present invention, a seal assembly is provided that is disposed in a groove in the inside surface of a hollow first member, with the inner edge of a seal member contacting a cylindrical second member disposed inside the first member to form a seal. The seal assembly is of similar construction to that discussed above.
In further accordance with the present invention, a barrel-shaped piston is provided for use in a hydraulic actuator of the type including a cylinder, the piston movably disposed in the cylinder, and a piston rod assembly connected to the piston and extending from an end of the cylinder. A seal assembly is disposed in an annular groove at or near the midpoint of the piston. The piston is tapered towards each end from each edge of the groove, such that the diameter of the piston at each edge of the groove is greater than the diameter at each respective end of the piston. The piston""s axial alignment can differ from that of the cylinder while allowing movement of the piston and maintaining the seal between the piston and the cylinder.
A hydraulic actuator is also provided that includes a cylindrical piston disposed in a casing with an hourglass-shaped inside surface. A seal assembly is disposed in an annular groove in the casing. The casing is tapered from each end toward each edge of the groove, such that the diameter of the casing at each edge of the groove shoulder is less than the diameter at each respective end of the casing. Again, the piston""s axial alignment can differ from that of the casing while allowing movement of the piston and maintaining the seal between the piston and the casing.
In further accordance with the present invention, several active mounts for mounting a vibrating component to a support structure are provided for use in a system for reducing vibration transmission from the vibrating component to the support structure. Each active mount includes a housing adapted to be attached to one of the vibrating component or the support structure, and a hydraulic actuator disposed in the housing. The actuator comprises a cylinder, a piston movably disposed in the cylinder, and a piston rod assembly connected to the piston and extending from an end of the cylinder. In one active mount that is provided, a seal assembly according to the present invention as described above is disposed either in a groove in the piston or in a groove in the cylinder, and includes an annular seal member comprising polytetrafluoroethylene (PTFE) resin. One edge of the seal member is disposed in the groove, and the seal member""s other edge contacts whichever one of the piston or cylinder that does not have the groove. The seal assembly also includes backing rings on each side of the seal, and on one side, an annular compression spring in the groove between one of the backing rings and one edge of the groove. The spring exerts force axially to compress the seal member, impelling the seal member to increase in dimension radially and maintain the seal between the piston and cylinder.
In another active mount that is provided, the actuator includes a barrel-shaped piston according to the present invention as described above, with a seal assembly disposed in an annular groove at or near the midpoint of the piston. Similarly, an active mount is provided where the actuator includes a cylindrical piston disposed in a casing with an hourglass-shaped inside surface, according to the present invention as provided above. A seal assembly is disposed in an annular groove in the casing. In both of these mounts, the piston""s axial alignment can differ from that of the casing while allowing movement of the piston and maintaining the seal between the piston and the cylinder or casing.
In yet further accordance with the present invention, an active mount for mounting a transmission gearbox to an airframe of a rotary wing aircraft is provided that comprises a housing adapted to be attached to one of the gearbox or the airframe, and a hydraulic actuator disposed in the housing. The actuator includes a cylinder, a piston movably disposed in the cylinder, and a piston rod assembly connected to the piston and extending from an end of the cylinder. A seal assembly is disposed either in a groove in the piston or in a groove in the cylinder, and includes an annular seal member comprising PTFE resin. One edge of the seal member is disposed in the groove, and the seal member""s other edge contacts whichever one of the piston or cylinder that does not have the groove. The seal assembly also includes backing rings on each side of the seal, and on one side, an annular compression spring in the groove between one of the backing rings and one edge of the groove. The spring exerts force axially to compress the seal member, impelling the seal member to increase in dimension radially and maintain the seal between the members.
The present invention features a seal assembly with backing rings that increase the stiffness of a PTFE seal and a compression spring that impels that PTFE seal to increase in radial dimension. The spring allows for volume expansion of the seal with temperature changes, without increasing the contact pressure between the seal and cylinder wall. This design for the seal also differs from convention in the art in that the seal is confined in all directions within the piston""s groove. The PTFE seal is preferably bonded to one or more of the backing rings to further increase circumferential stiffness. A barrel-shaped piston or hourglass-shaped casing accommodate transverse movement between the support structure and mounted component. An elastomeric thrust bearing is part of the piston rod assembly, and is stiff along the piston rod assembly""s thrust axis but flexible to transverse and rotational motions.
The seal assembly of the present invention provides a low-friction seal that minimizes leakage of hydraulic fluid. The seal has adequate radial stiffness to maintain clearance between the actuator""s piston and cylinder when subjected to transverse loads. Cocking of the piston is permitted without binding by tapered walls of the piston or casing. Accordingly, transverse motions and rotations at the points of attachment do not induce high loads between the piston and cylinder, and the piston will not bind in the cylinder. The seal assembly maintains the seal between the piston and cylinder even when the piston is cocked. The hydraulic actuator is compact and lightweight. In an active mount system, the actuator will provide maximum reduction of noise and vibration transmission between a vibrating component and a support structure. The actuator is well suited to incorporation in systems such as on a mount for a helicopter main rotor.
The foregoing and other features and advantages of the present invention will become more apparent in light of the following detailed description of the preferred embodiments thereof, as illustrated in the accompanying figures. As will be realized, the invention is capable of modifications in various respects, all without departing from the invention. Accordingly, the drawings and the description are to be regarded as illustrative in nature, and not as restrictive.